Pulmonary surfactant (PS), a complex mixture of lipids and proteins that forms an extremely thin (0.01 um) layer on the inner lining of the lung airways and alveoli (air-liquid interface), has the capacity to reduce surface tension of the air-liquid interface to nearly 0 dynes/cm. It enhances lung compliance and prevention of alveolar collapse and alveolar flooding. In mammals including humans, high proportions (50 percent) of the PS consists of saturated phospholipids, whose capacity to reduce surface tension are greatly compromised by body temperatures lower than 37 degrees C because saturated phospholipids undergo a structural phase change (gel to liquid crystalline) below normal body temperature. While this may not seem problematic for normothermic individuals, this relationship between surfactant function and body temperature in hypothermic individuals has not previously been recognized, and has direct implications in potential causes of Sudden Infant Death Syndrome (SIDS), the clinic treatment of accidental hypothermia, and the current widespread use of induced hypothermia for neuro and cardiovascular surgery. In contrast to human, certain mammal species (temperate bats, deer mice) display daily reductions in body temperatures (voluntary hypothermia or torpor) that are considered severely lethal by human standards, without any sign of respiratory distress. The capacity for rapid recovery from hypothermia in these animals strongly suggests an adaptation of their PS system to body temperature change (e.g., rapid adjustment of saturated/unsaturated phospholipid ratio, incorporation of cholesterol). The overall objective of the proposed study is to compare the PS system of a traditionally homeothermic mammal species (lab rat) with that of a renown heterothermic mammal (big brown bat). Specific goals of this comparison are to 1) measure the PS composition under normothermic and hypothermic conditions, 2) determine how hypothermia influences lung compliance, and 3) describe the surface tension properties of PS at a variety of temperatures in vitro. Compositional analysis of lavaged PS will focus on the degree of saturation of phospholipids and the amount of cholesterol. How the PS of each species responds to hypothermia to possibly compromise lung inflation and deflation will be measured by specific compliance. In vitro characterization of the surface tension properties of PS as affected by temperature will be evaluated using balance surfactometry and captive bubble analysis.